1. Field of the Invention
The present invention relates to a developing method for forming an image using a developer including a toner and a magnetic carrier. In addition, the present invention also relates to a developing device, and a process cartridge and an image forming apparatus including a developing device.
2. Discussion of the Background
In recent years, image forming apparatus such as copiers and laser printers are required to produce high quality images while having a good combination of durability and stability. Specifically, image forming apparatus are required to stably produce high quality images for a long period of time even when environmental conditions are changed.
On the other hand, two component developing methods have been broadly used for electrophotographic image forming apparatus. Two component developing methods typically include the following steps:    (1) forming a magnetic brush of a two component developer including a toner and a magnetic carrier on a developer bearing member (such as a developing sleeve) by means of the magnetic poles of a magnet included in the developer bearing member; and    (2) developing an electrostatic latent image formed on an image bearing member with the magnetic brush in a developing region in which the image bearing member and the developer bearing member are opposed to each other while applying a developing bias to the developer bearing member.
Such two component developing methods have an advantage in that color images can be easily produced.
In the two component developing methods, the developer is fed to the developing region by rotation of the developing sleeve. In this regard, when the developer is fed to the developing region, particles of the magnetic carrier in the developer layer on the developing sleeve are gathered by means of the magnetic pole (i.e., the developing pole) while the carrier particles accompany the toner particles, resulting in formation of the magnetic brush.
As described in published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 06-194961, an alternate electric field such that an electric field which causes toner particles to move toward the image bearing member and another electric field which causes the toner particles to move toward the developer bearing member are alternately applied is typically used as the developing bias. By applying such an alternate electric field, a high developability can be imparted to the developing device, and images having high image density can be stably produced. In addition, even when the charge quantity distribution of the toner used for the two component developer is changed after long repeated use, images having high image density can be stably produced. Further, even when light half tone electrostatic images (i.e., half tone electrostatic images corresponding to half tone toner images having a low image density) are developed, proper amounts of toner particles can be adhered to the half tone electrostatic images by applying such an alternate electric field. Thus, by using such an alternate electric field, not only high developing ability can be imparted to the developing device but also half tone electrostatic images can be faithfully reproduced. Therefore, the developing method using an alternate electric field is typically used for color image forming apparatus. In addition, when the developing method is used for monochrome image forming apparatus, half tone images with little granularity (i.e., half tone images having good evenness) and solid images with good evenness can be produced. Therefore, the developing method is suitable for not only color image forming apparatus but also monochrome image forming apparatus.
However, when an alternate electric field is used as the developing bias, a problem which occurs is that a white ring image is formed in the resultant toner image due to occurrence of discharging in a portion of an electrostatic latent image corresponding to a half tone toner image having a high image density, which is caused by local increase of electric field strength in a magnetic brush having a dense portion and a thin portion. In order to avoid such a white ring image problem, the resistance of the carrier is preferably increased. However, even when the resistance of the carrier is optimized, occurrence of discharging cannot be prevented if a layer (such as a resin layer) formed on the carrier particles has uneven thickness. This is because the portions of the carrier particles having a thin resin layer thereon cause breakdown. Thus, in order to produce images having good evenness, various conditions (such as the resistance of the carrier and thickness of the coated layer formed on the carrier) have to be controlled.
In attempting to prevent formation of granular images (i.e., images with poor evenness) even when the developing method using an alternate electric field is used, various studies have been made. It is well known as a result of the studies that formation of granular images can be prevented by accelerating rearrangement of toner particles when the toner particles are used for developing an electrostatic image.
However, the developing methods using an alternate electric field have the following drawbacks:    (1) the maximum value of the electric field is relatively large compared to a case where a direct electric field is formed, and thereby a problem in that carrier particles are adhered to electrostatic latent images is easily caused; and    (2) an additional power source is necessary for forming an alternate electric field, and thereby the manufacturing costs of the developing device increase.
Therefore, various studies have been made for preventing occurrence of granular images even when a direct electric field is used as the developing bias.
It is well known as a result of various studies that granular images are formed because a thin magnetic brush is formed in the developing region. In attempting to prevent formation of such granular images (i.e., to produce high quality images), JP-A 08-146668 discloses a developing method in which the volume ratio of the carrier particles included in the magnetic brush in the developing region is specified. However, as a result of the present inventor's study, it is found that the granularity of images changes even when the volume ratio of carrier particles in a developing region is controlled to be uniform.
Because of these reasons, a need exists for a developing method, a developing device and an image forming apparatus by which high quality images with little granularity and good dot reproducibility can be produced.